1. Field of the Invention
The present invention relates to power filters in general, and to active power filters in particular.
2. Description of the Prior Art
The wide use of nonlinear loads has increased the harmonic content of the voltage and current waveforms in alternating current (AC) power distribution systems. In many cases large numbers of such loads are operating, causing a corresponding increase in power line harmonics. Such harmonic currents in conjunction with their associated source impedances produce distortion of the line voltages which can cause equipment to malfunction.
To address the above problems, active power filters (APF) have been used for compensation of polluting harmonics on electricity distribution networks. An APF is a device that is connected to a power line and cancels the reactive and harmonic currents from a group of nonlinear loads so that the resulting total current drawn from an AC source is sinusoidal. Ideally, the APF needs to generate just enough reactive and harmonic current to compensate the nonlinear loads on the line, thus it handles only a fraction of the total power to the loads.
In one conventional APF design, an open loop scheme is used to control the APF current, as generally described by E. Dallago and M. Passoni in an article titled “Single-Phase Active Power Filter with Only Line Current Sensing”, IEEE Electronics Letters, 20 Jan. 2000, Vol. 36, No. 2, pp. 105-106, and by K. M. Smedley, L. Zhou, and C. Qiao in an article titled “Unified Constant Frequency Control of Active Power Filters-Steady State and Dynamics”, IEEE Transactions on Power Electronics, Vol. 16, No. 3, May 2001, pp. 428-436. In these systems the APF is controlled such that the output voltage of the APF is proportional to the input source current. This causes the impedance seen by the power source to appear resistive, hence maintaining the input current approximately proportional to the input voltage.
In another conventional APF design, a closed input current loop with a reference multiplier is used, as described by F. P. de Souza and I. Barbi in an article titled “Single-Phase Active Power Filters for Distributed Power Factor Correction”, IEEE PESC 2000, pp. 500-505. A current loop is used to force the input source current to track the input source voltage, thereby achieving near-unity power factor.
However, in the above approaches, only the input source current is measured. This makes it difficult to control the current flow from the APF and the load. Hence, if there is an overload condition at the load, the APF control will attempt to deliver the current demanded, even if the demand is beyond the design limits of the APF.
There is, therefore, a need for an APF control method and apparatus that limit the APF current, and allow transition from a normal condition to an overload condition and back again in stable manner, while maintaining the lowest input current distortion within the design limits of the APF.